This invention relates to a car display unit and in particular to a car display unit for displaying a road map screen and various menu screens in car navigation.
The technical background of the invention will be discussed. Generally, it is known that an image is inverted depending on the screen viewing angle with a car display unit. It is assumed that inversion of the image occurs as contrast changes with the viewing angle and that the contrast changes with the signal voltage (signal level) of a video signal (R, G, B signal) of an input signal to a liquid crystal display (LCD). The relationships between input signal voltage and contrast when a liquid crystal display 1 is viewed from direction A obliquely relative to the liquid crystal display 1, direction B directly above the liquid crystal display 1, and direction C obliquely relative to the liquid crystal display 1, for example, as shown in FIG. 14, become as shown in FIGS. 15A, 15B, and 15C. As seen in FIG. 15A, if a predetermined input signal voltage is exceeded, the contrast again rises and image inversion occurs in the direction A relative to the liquid crystal display 1.
FIG. 16 is a graph to show the relationship between applied voltage to liquid crystal of each pixel part and transmittance of light of backlight passing through the pixel part when the liquid crystal display 1 is viewed from the direction A. Here, the applied voltage to liquid crystal of each pixel part is changed with the signal voltage of a video signal input for each pixel.
FIG. 16 is applied to normally white liquid crystal display 1; basically, as the applied voltage increases, the light transmittance lowers and screen display darkens. When the liquid crystal display 1 is viewed from the direction A, the light transmittance bounds in the latter half portion of the change range in which the light transmittance lowers gradually with an increase in the applied voltage, and the area in which the applied voltage to liquid crystal can be changed contains an image inversion area 3 in which image inversion occurs depending on the visible direction.
In FIG. 16, numeral 5 denotes a standard change range of applied voltage to liquid crystal changed with the signal level of a video signal. In the standard change range 5, a gradation area less than a halftone area in the change range 5 overlaps the image inversion area 3 and image inversion occurs in the overlap area.
The change range 5 is set from the two viewpoints of suppression of the effect of image inversion and providing sufficient contrast; the gradation area less than the halftone area in the change range 5 largely overlaps the image inversion area 3. Thus, to display a screen with gradation level (brightness level indicated by the signal level of a video signal) set low as a whole (comparatively dark display screen), the effect of image inversion is largely received.
By the way, a car display unit provided for a car navigation system, etc., changes different screens such as road map screens and various operation screens (menu screens) for setting a destination, etc., based on an input signal input through a predetermined input part such as a touch switch. The screens may differ from each other in brightness level fitted to display because of the difference in the display form, etc., of displayed visible information, and the effect of image inversion described above also varies from one screen to another.
In contrast, the conventional car display unit displays such different screens by changing applied voltage to liquid crystal in the liquid crystal display in the single standard change range 5; the different screens are displayed at the same brightness level.
To decrease the effect of image inversion, etc., the change range of applied voltage to liquid crystal may be adjusted for correcting the brightness level depending on the screen type, as described above. However, the conventional car display unit basically displays the different screens at the same brightness level, thus the effect of image inversion is largely received depending on the screen, and visibility is degraded.
Some of such car display units are provided with a brightness control, etc., for changing the change range of applied voltage to liquid crystal for adjusting the screen brightness level, but it is difficult for the user to operate the brightness control, etc., each time in association with screen change made comparatively frequently.